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D. Chen, R. Bremner; The Mechanism of Cone Loss and Other Phenotypes in the Rb/p107–Deficient Retina . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4187.
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© ARVO (1962-2015); The Authors (2016-present)
Rb/p107 loss causes ectopic division and apoptosis of cones and other cell types as well as tumorigenesis in the developing mouse retina. Rb can quench the ability of the activating E2Fs (E2F1–3) to drive cell division. Our goal was to determine whether unleashed E2F1, 2 and/or 3 drive cone loss and other phenotypes in the Rb/p107–deficient mouse retina.
Cre recombinase can be used to delete genes that have been engineered to contain two flanking loxP sites. We crossed mice that have a floxed Rb allele with another strain carrying a Cre recombinase transgene (alpha–Cre), which is active in peripheral retinal progenitors at embryonic day 10. These mice were also crossed with animals lacking the Rb–related gene p107. Finally, because Rb and its relatives inhibit gene activation by E2F proteins, we asked whether defects observed in Rb– or Rb/p107–deficient mice could be rescued by inactivating E2Fs. E2F1 and p107 are only 2Mb apart so an extensive breeding program was set up to obtain the crossover necessary to generate the E2F1–;p107– haplotype.
Retinal development involves expansion of retinal progenitor cells (RPCs), specification of differentiating post–mitotic retinal transition cells (RTCs) and terminal differentiation. Previously we found that inactivation of Rb and p107 does not affect either RPC division rate or specification of RTCs, but causes ectopic division of all RTCs. Ectopically dividing ganglion, bipolar, cone and rod RTCs undergo apoptosis, and retinoblastoma arises from death resistant amacrine RTCs. Here, we show that deletion of E2F1, but not E2F2 or E2F3 blocks tumorigenesis and most ectopic RTC division, as well as apoptosis of rod, bipolar and ganglion cells in the Rb/p107 deficient retina. However, ectopic division of cone RTCs and their subsequent loss was not reversed in the Rb/p107/E2F1 triple knockout (TKO) retina. In stark contrast, many cones were evident in the Rb/p107/E2F2 TKO. They survived up to 3–4 weeks after birth, but were gone by postnatal day 60. Remarkably, inactivation of E2F1–3 ensured long–term cone survival in the Rb/p107–deficency retina.
In the absence of Rb and p107, E2F1 drives ectopic division of all Rb/p107–deficient RTCs and is essential for tumorigenesis. Unleashed E2F1 activity specifically drives the death of rod, bipolar and ganglion RTCs. In contrast, E2F2 is the major cause of cone RTC loss. However, deletion of all three activating E2Fs is required to ensure long–term survival of Rb/p107–deficient cones. These data highlight cell–specific roles of distinct activating E2Fs in the Rb/p107 deficient retina.
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